The present invention relates generally to high temperature protection for aircraft structures, and more particularly to active cooling systems for hypersonic vehicle nose caps and leading edges.
Nosecaps and leading edges of hypersonic (also called hypervelocity) vehicles can experience uncooled temperatures surpassing 6000.degree. F. These high temperatures require the use of either active or passive cooling systems to maintain an acceptable structural temperature. Two of the more common active cooling methods are transpiration and forced convection. Transpiration cooling systems inject coolant into the atmosphere through holes distributed over the outer skin of a hypersonic vehicle. Cooling results from the phase change from liquid to gas of the transpirant coolant and from film cooling, in which the resulting gas film deflects the hot boundary layer away from the outer skin of the hypersonic vehicle. Forced convection cooling systems pump coolant through passages below the outer skin of the hypersonic vehicle. Cooling results from the convection coolant absorbing heat from the outer skin of the hypersonic vehicle. The hot convection coolant is then pumped to a heat exchanger, where it gives up the heat it absorbed from the outer skin, and then returns to the passages below the outer skin to absorb more heat.
Each of these two methods requires different tradeoffs among weight, reliability and safety. Transpiration cooling systems, because of their use of expendable coolants, are typically much heavier than forced convection cooling systems. Nevertheless, they generally are more efficient, providing the most cooling for the least weight of coolant. For identical reentry conditions, the coolant requirements for transpiration cooling systems will be much less than that for forced convection cooling systems. Moreover, the safety and reliability of transpiration cooling systems have been demonstrated by their successful use for high temperature protection in the past.
Forced convection methods, although never used in a hypersonic environment, have an advantage of their own in that they can use the liquid hydrogen or other hypersonic vehicle fuel as their coolant, so that the weight of the coolant as a design parameter is largely removed. Transpiration cooling systems have a similar advantage in that, for reentry, they can use as the transpiration coolant water created in orbit as a byproduct of power generation.
It is seen, therefore, that while transpiration cooling systems are generally more efficient than forced convection cooling systems, each cooling system has separate unique advantages particularly relevant to a hypersonic vehicle.
It is, therefore, a principal object of the present invention to provide an improved apparatus and method for high temperature protection for hypersonic vehicle nosecaps and leading edges that integrates the best features of both transpiration and forced convection cooling systems with as few as possible of their disadvantages.
It is another object of the present invention to make a hypersonic vehicle cooling system that is both safer and significantly lighter than prior cooling systems.
It is a feature of the present invention that it provides redundancy in the event of a partial failure of the cooling system.
It is an advantage of the present invention that its hybrid design will be generally safer and significantly lighter than either a transpiration or forced convection system used alone.
It is another advantage of the present invention that it will be more reliable than either cooling system used alone.
It is a further advantage of the present invention that its operation can be optimized to reduce the required expendable onboard coolant, reduce thermal stresses, and allow vehicle operation at higher dynamic pressures than are now planned, significantly improving engine performance.
It is yet another advantage of the present invention that it will allow for more extreme temperatures during envelope expansion of a hypersonic vehicle than either cooling system used alone.
It is also an advantage of the present invention that it is straightforward and easy to implement.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.